There is a road right of the big door leading inside the building. You will see a small opening, just right of where you killed the tanks. Proceed through there and make your way up in the house, killing the machinegun and the enemies on your way. Then enter the house, kill the remaining enemies and follow the path outside. From there cross the bridge hanging over the large door, and enter the small room that contains the second door mechanism. Place a C4 charge, blow it and then the doors will open and a tank will come out... Good luck